Bug 1493081 - Added nsIWidget::GetDesktopToDeviceScaleByScreen for scale factor lookup by window position; r=mattwoodrow
We've added nsIWidget::GetDesktopToDeviceScaleByScreen which will return scale factor of the newly placed window
according to its position on the display. This change is to move implementation to the nsIWidget derived classes.
We need that for GTK Wayland, because on the Wayland we cannot determine absolute position of the window, we
need to use parent's window scale factor. For other platforms the GetDesktopToDeviceScaleByScreen is implemented
in nsBaseWidget.
Differential Revision: https://phabricator.services.mozilla.com/D7290